Method for bleaching and color stripping recycled fibers

ABSTRACT

A method for bleaching and color stripping recycled fibers by treating the fibers with a mixture of sodium borohydride and sodium bisulfite.

[0001] This invention relates generally to a method for bleaching andcolor stripping recycled fibers with sodium borohydride and sodiumbisulfite.

[0002] One type of reductive bleaching process involving sodiumborohydride and sodium bisulfite for bleaching recycled fibers isdisclosed in U.S. Pat. No. 5,562,803. This reference describes asequential addition of sodium bisulfite and sodium borohydride in atwo-stage process, with sodium bisulfite being added first, followed byaddition of sodium borohydride to generate hydrosulfite. This process isreferred to as the “DBI” process.

[0003] The problem addressed by this invention is to find a moreefficient process using bisulfite and borohydride for reductivebleaching and color stripping of recycled fibers.

STATEMENT OF THE INVENTION

[0004] This invention is directed to a method for bleaching and colorstripping recycled fibers by treating the fibers with a mixture ofsodium borohydride and sodium bisulfite.

[0005] In a first embodiment of the invention, the method comprises thesteps of: (a) adding sodium bisulfite to a recycled fiber slurry toproduce a bisulfite-treated slurry; and (b) adding to thebisulfite-treated slurry a mixture produced by combining: (i) an aqueoussolution comprising sodium borohydride and sodium hydroxide; and (ii) anaqueous solution comprising sodium bisulfite.

[0006] In a second embodiment of the invention, the method comprisescombining: (i) an aqueous solution comprising sodium borohydride andsodium hydroxide; and (ii) an aqueous solution comprising sodiumbisulfite, to produce a combined solution, and adding the combinedsolution to an aqueous slurry of recycled fibers; wherein the ratio of(moles bisulfite - moles hydroxide)/moles borohydride is from 0 to 25.

DETAILED DESCRIPTION OF THE INVENTION

[0007] All percentages are expressed as weight percentages, unlessspecified otherwise. The term “recycled fibers” refers to paper that hasbeen subjected to reuse for the purpose making paper or paper productsagain. The term “pre-mix” refers to a process in which borohydride andbisulfite are mixed prior to addition to the pulp.

[0008] Dithionite ion, also referred to as hydrosulfite, can be producedby the reaction between bisulfite and borohydride ions, according to thefollowing theoretical equation:

BH₄ ⁻+8HSO₃ ⁻+H⁺→4S₂O₄ ⁻²+B(OH)₃+5H₂O

[0009] The yield is somewhat less than 100% due to competing reactions,including that of borohydride with water, but is most often better than85%. Since the exact mechanism of the reaction has not been fullycharacterized, this invention is not limited to reduction by dithioniteion, and other species present in the reaction mixture also may act asreducing agents. When the amount of bisulfite is at 8 moles per mole ofborohydride, the theoretical reaction proceeds to completion. Withoutwishing to be bound by theory, it is believed that use of more than thetheoretical amount of bisulfite or less than the theoretical amount ofbisulfite results in a mixture containing hydrosulfite, sodiumbisulfite, borohydride and possibly other species.

[0010] In the first embodiment of the invention, in which sodiumbisulfite pre-treatment is performed, the pre-treatment is carried outby adding the bisulfite to the recycled pulp slurry to maintain a pH ofthe slurry between 5.0 and 8.5. Preferably, the pH is no more than 8.0,more preferably no more than 7.8, and most preferably no more than 7.5.Preferably, the pH is at least 5, more preferably at least 5.5, and mostpreferably at least 6.0. Preferably, the amount of bisulfite added tothe pulp slurry as a pre-treatment, measured as the percentage of sodiumbisulfite relative to the dried fiber content of the pulp, is at least0.025%, more preferably at least 0.15%, and most preferably at least0.25%. Preferably, the amount of bisulfite added to the pulp slurry,measured as the percentage of sodium bisulfite relative to the driedfiber content of the pulp, is no more than 5%, more preferably no morethan 3%, more preferably no more than 2% and most preferably no morethan 1%. In a preferred embodiment of the invention, a 38% aqueoussodium bisulfite solution is used. Use of bisulfite pre-treatment ispreferred for lower-brightness fibers, i.e., fibers having an ISObrightness less than 75%, and especially for fibers at less than 65%.For lower-brightness fibers, preferably at least 0.15% bisulfite isadded for pre-treatment.

[0011] In a preferred embodiment of the invention, borohydride is addedin the form of an aqueous solution containing sodium borohydride andsodium hydroxide. Also aqueous sodium bisulfite solution is mixed withaqueous solution containing sodium borohydride and sodium hydroxide. Inthis embodiment, some of the bisulfite is consumed in a neutralizationreaction with the hydroxide ion. In some applications, hydroxide ionpresent in borohydride solutions is neutralized by acid added to thebisulfite solution. In such a case, to the extent that the hydroxideinitially present in the borohydride solution has been neutralized, itwill not consume bisulfite, and will not be included in the ratiocalculation. As described above, the theoretical reaction of borohydrideand bisulfite requires 8 moles of unconsumed bisulfite per mole ofborohydride, i.e., the ratio (moles bisulfite-moles hydroxide)/molesborohydride is approximately 8. The present invention uses a ratio from0 to 25. Preferably, the ratio is no more than 23, more preferably nomore than 20, and most preferably no more than 18. Preferably, the ratiois at least 2, more preferably at least 3, and most preferably at least4.

[0012] In the first embodiment of the invention, in which the fiberslurry is pre-treated with sodium bisulfite, followed by borohydride andbisulfite solutions which are combined and then added to the slurry, theratio (moles bisulfite-moles hydroxide)/moles borohydride combined foraddition to the pulp (i.e., excluding the bisulfite used forpre-treatment) preferably is at least 3, more preferably at least 5,more preferably at least 6, and most preferably at least 7; preferablythe ratio is no more than 15, more preferably no more than 14, morepreferably no more than 13, and most preferably no more than 12. Thetime interval between addition of sodium bisulfite and addition of thepre-mixed solutions is not believed to be critical. Preferably, a timeinterval sufficient to ensure thorough mixing of the bisulfite and thepulp is allowed, preferably at least 30 seconds, more preferably atleast one minute, more preferably at least five minutes, and mostpreferably at least ten minutes. Preferably, the time interval is nolonger than 5 hours, more preferably no longer than 2.5 hours, and mostpreferably no more than 1 hour.

[0013] In the second embodiment of the invention, in which there is nosodium bisulfite pre-treatment, the ratio (moles bisulfite-moleshydroxide)/moles borohydride preferably is from 2 to 7 or from 9 to 16,more preferably from 2 to 6 or from 10 to 15, and most preferably from 2to 5 or from 11 to 15. The data provided below in the Examplesdemonstrates, unexpectedly, improved performance at lower or higher thanthe theoretical 8 moles of unconsumed bisulfite per mole of borohydride.

[0014] In one embodiment of the invention, combining water and sodiummetabisulfite, Na₂S₂O₅, generates bisulfite. The aqueous sodiumbisulfite preferably is about 2% to about 45% active by weight. Apreferred borohydride composition for use in accordance with the methodsof the invention is in liquid form and comprises about 1% to about 36%active sodium borohydride and about 30 to about 40% NaOH or Na₂CO₃ (alsoknown as soda ash), all by weight. A particularly preferred borohydridecomposition comprising 12% active sodium borohydride and 40% NaOH iscommercially available from Rohm and Haas Company under the trademarkChromaClear™ solution. (For example, 100 g of ChromaClear™ solutioncontains 12 g sodium borohydride, 40 g NaOH, and 48 g H₂O). When thesodium borohydride solution contains sodium hydroxide, e.g.,ChromaClear™ solution, the theoretical equation for reaction withbisulfite is as follows:

[0015] [NaBH₄+3.2 NaOH]+11.2 NaHSO₃→4 Na₂S2O₄+(NaBO₂+3.2 Na₂SO₃+9.2H₂O)

[0016] In this case, where there are 3.2 moles of hydroxide per mole ofborohydride, and the hydroxide has not been neutralized with a mineralacid, the ratio of bisulfite unconsumed by hydroxide to borohydride is(11.2−3.2)/1=8.0, i.e., the theoretical ratio.

[0017] In the first embodiment of this invention, preferably, theaddition of sodium bisulfite for pretreatment is carried out by addingbisulfite solution to the pulp slurry after the slurry has beenscreened, cleaned, thickened and made ready for papermaking i.e., afterthe deckers in a typical pulp mill. In one preferred embodiment of theinvention, the bisulfite is added to the MC standpipe in which pulpslurry accumulates prior to being pumped to the up-flow tower or thechest. Preferably, the total of the sodium bisulfite added forpre-treatment and the sodium bisulfite combined with sodium borohydrideis at least 0.05%, more preferably at least 0.1%, more preferably atleast 0.2%, and most preferably at least 0.5%; preferably the totalamount is no more than 5%, more preferably no more than 3%, and mostpreferably no more than 2%.

[0018] In both the first and second embodiments of this invention, aborohydride solution and a bisulfite solution are combined to produce amixture which is then added to the pulp slurry. Preferably, thesolutions are combined by mixing them in an in-line static mixer, aT-pipe mixer, a pump recirculator, an agitator, the suction side of apump impeller, a high-shear mixer, or a low-shear mixer. Mostpreferably, the mixer is an in-line static mixer. Typical in-line staticmixers have from 2 to 24 internal elements, preferably from 2 to 6internal elements. The length of the piping from the mixer to the pointof addition to the pulp slurry also may affect the mixing; preferablythis length is at least 3.28 ft (1 m), more preferably at least 4.92 ft(1.5 m). The number of elements, the diameter of the mixer and thelength of piping required to achieve good mixing, i.e., to produce asubstantially homogeneous mixture, can be determined easily from theflow parameters and fluid properties of each particular system. Forexample, in one method dye is added to one of the solutions and goodmixing is assessed by visible determination that the color of the outputis uniform. In another method, the pH of the pulp slurry after additionof the mixed borohydride and bisulfite solutions is measured; a stablepH value is an indication of good mixing, as are consistent bleachingresults. Preferably, if mixing is insufficient, the borohydride andbisulfite solutions are diluted. Preferably, the borohydride andbisulfite solutions are mixed at a temperature in the range from 4° C.(39.2° F.) to 50° C. (122° F.), more preferably from 10° C. (50° F.) to35° C. (95° F.).

[0019] Preferably, the mixed borohydride and bisulfite solutions areadded to the pulp slurry directly, or by storing the output in a vesselfor later addition to the pulp slurry. In one preferred embodiment, theoutput of the mixer is stored in a vessel and added to the pulp slurrywithin 12 hours of mixing, more preferably within 6 hours, morepreferably within 3 hours, more preferably within 1 hour, and mostpreferably within ½ hour of mixing. In another preferred embodiment, themixer output is added directly through piping which carries the outputto the pulp slurry in less than 15 minutes, more preferably less than 10minutes, and most preferably less than 5 minutes.

[0020] Preferably, the amount of borohydride combined with sodiumbisulfite, measured as the percentage of sodium borohydride relative tothe dried fiber content of the pulp, is at least 0.003%, more preferablyat least 0.006%, and most preferably at least 0.009%. Preferably, theamount of borohydride, measured as the percentage of sodium borohydriderelative to the dried fiber content of the pulp, is no more than 0.12%,more preferably no more than 0.08%, and most preferably no more than0.04%. In a preferred embodiment of the invention, a 12% aqueous sodiumborohydride solution is used, e.g., ChromaClear™ solution. In thisembodiment, the weight of the solution used, measured as a percentage ofthe dried fiber content of the pulp, is at least 0.025%, more preferablyat least 0.05%, and most preferably at least 0.075%. Preferably, theweight of solution used, measured as a percentage of the dried fibercontent of the pulp, is no more than 1%, more preferably no more than0.67%, and most preferably no more than 0.33%.

[0021] Preferably, the mixed borohydride and bisulfite solutions thatare the output of the mixer are added to the pulp slurry afterpre-treatment of the pulp slurry with bisulfite. In another preferredembodiment, the mixed solutions containing higher/lower thanstoichiometric amount of bisulfite are added to fibers in thede-fibering stage, for example in the refiners or grinders or in theabove locations without any bisulfite pretreatment.

EXAMPLES Example 1

[0022] Laboratory Studies

[0023] Table 1 shows the results of a comparison between pre-mix andbisulfite pre-treated pre-mix processes on a sorted office wastefurnish. Both pre-mix and bisulfite-pretreated pre-mix were performedwith the solutions of bisulfite and borohydride combined at a bisulfiteto borohydride molar ratio of 13.3:1. Bisulfite pretreatment wasconducted at 10 lb/ton (0.5% based on dry fiber) of sodium bisulfite,followed immediately after thorough mixing of the bisulfite into thepulp by pre-mix addition to the pre-treated pulp. Bleaching conditionsare listed in Table 1. It is clear that bisulfite-pretreated pre-mixperformed significantly better than pre-mix especially at lower pre-mixtreat rates. Table 2 shows the results of pre-mix at 12:1, 15:1, and18:1 bisulfite to borohydride molar ratios on a mixed office wastefurnish. Bleaching conditions can be seen in Table 2. Pre-mix at thehigher ratios of 15 and 18 performs better than pre-mix at the lowerratio of 12. Table 3 shows the results of pre-mix at 2.9:1 and 12:1bisulfite to borohydride molar ratios on a sorted color furnish.Bleaching conditions can be seen in Table 3. It is clear that in thiscase, pre-mix at lower SBS ratio of 2.9:1 unexpectedly performs betterthan pre-mix at higher SBS ratio of 12:1. Table 4 compares the pre-mixprocess to the DBI process at various bisulfite to borohydride ratios ona mixed office waste furnish, and demonstrates that the pre-mix processperforms better than the DBI process.

[0024] Pre-mix Bleaching Process

[0025] Based on consistency, 7 g O.D. pulp was placed in heavy gaugepolyethylene bags. The bags were sealed under nitrogen, shakenvigorously to disperse the pulp fiber, and preheated in a constanttemperature bath to get to the desired bleach temperature. Pre-mixsolutions were generated from ChromaClear™ solution and NaHSO₃ (SBS). Inthe method of generating the pre-mix solution, sodium bisulfite powderwas added to water in a round bottom flask and stirred until the sodiumbisulfite powder had completely dissolved. ChromaClear™ solution wasthen immediately added under an inert atmosphere and under rapidstirring to generate a completely formed pre-mix solution. Based on theborohydride concentration of each solution, the required volume ofpre-mix solutions was calculated. Bleaching was carried out by addingthe pre-mix solution under nitrogen and keeping the pipette below thesurface of the pulp.

[0026] Each bag was resealed, shaken thoroughly to mix, and returned tothe constant temperature bath for the desired time. At the end of thebleaching period each bag was removed from the bath and the pH wasmeasured and recorded. The pulp was then diluted to 1% using deionizedwater. One handsheet was made from each run and air dried overnight at50% relative humidity. Brightness readings were measured using aTechnibrite™ ERIC 950 and are the average of five readings from each 7 gO.D. handsheet. TABLE 1 Laboratory bleaching response of pre-mix andbisulfite pretreated pre-mix processes. SBS:CC Mole pH pH Bright.Process CC, % SBS, % Ratio¹ Initial Final % ISO L* a* b* No — — 9.1 —78.8 92.8 0.69 2.91 bleach Pre-mix 0.05 0.275 13.3 9.1 8.0 83.5 95.3−0.69 3.62 Pre-mix 0.1 0.55 13.3 9.1 7.6 84.6 95.6 −0.71 3.36 Pre-mix0.15 0.825 13.3 9.1 7.4 85.3 95.8 −0.78 3.10 Pre-mix 0.2 1.1 13.3 9.17.3 85.6 96.0 −0.80 3.25 Pre-SBS² + Pre-mix 0.05 0.275 13.3 7.1 7.2 85.295.6 −0.37 2.84 Pre-SBS² + Pre-mix 0.1 0.55 13.3 7.1 7.2 85.5 95.7 −0.432.84 Pre-SBS² + Pre-mix 0.15 0.825 13.3 7.1 7.1 85.6 95.7 −0.5 2.81Pre-SBS² + Pre-mix 0.2 1.1 13.3 7.1 7.1 86.0 95.8 −0.48 2.75

[0027] TABLE 2 Laboratory bleaching comparison at higher molar ratiobisulfite SBS:CC Mole pH pH Process CC, % SBS, % Ratio¹ Initial FinalBright. % ISO L* a* b* Pre-mix 0.25 1.25 12 7.5 7.0 63.8 87.8 −0.70 7.5Pre-mix 0.25 1.5 15 7.5 6.9 64.3 87.9 −0.80 7.5 Pre-mix 0.25 1.75 18 7.56.8 64.3 87.9 −0.90 7.5

[0028] TABLE 3 Laboratory bleaching comparison at lower and higher molarratio of (bisulfite - hydroxide)/borohydride SBS:CC Mole pH pH ProcessCC, % SBS, % Ratio¹ Initial Final Bright. % ISO L* a* b* No 5 5 22.361.2 11.8 11.1 Bleach Pre-mix 0.5 1 2.9 5 6.7 28.3 69.5 11.8 16.5Pre-mix 0.5 2.5 12 5 5.4 27.1 67.2 12.7 14.3

[0029] TABLE 4 Laboratory bleaching comparison of pre-mix and DBIprocesses SBS:CC Mole pH pH Process CC, % SBS % Ratio¹ Initial FinalBright. % ISO L* a* b* No — — 8.3 — 74.2 90.8 1.4 3.18 bleach DBI 0.050.20 8.9 8.40 8.42 74.6 91.2 1.01 3.52 0.10 0.40 8.9 8.40 8.23 78.2 92.80.41 3.32 0.15 0.60 8.9 8.40 8.05 79.3 93.1 0.36 3.12 0.20 0.80 8.9 8.407.90 80.1 93.4 0.31 2.91 0.30 1.20 8.9 8.40 7.90 80.7 93.5 0.31 2.650.40 1.60 8.9 8.40 7.70 81.2 93.6 0.33 2.42 DBI 0.05 0.35 18.0 8.54 8.0675.3 91.5 0.88 3.47 0.08 0.30 18.0 8.54 7.86 78.8 92.8 0.42 3.04 0.100.70 18.0 8.54 7.70 79.5 93.1 0.28 2.98 0.15 1.05 18.0 8.54 7.53 80.393.4 0.25 2.78 0.20 1.40 18.0 8.54 7.39 80.3 93.3 0.19 2.74 0.25 1.7518.0 8.54 7.30 80.5 93.4 0.15 2.72 0.30 2.10 18.0 8.54 7.22 80.6 93.40.11 2.72 Pre-Mix 0.050 0.20 8.9 8.40 7.90 78.1 92.7 0.41 3.36 0.0750.30 8.9 8.40 7.86 79.0 93.1 0.31 3.25 0.088 0.35 8.9 8.40 7.79 79.493.2 0.24 3.24 0.100 0.40 8.9 8.40 7.76 79.8 93.4 0.28 3.12 0.150 0.608.9 8.40 7.63 80.2 93.5 0.22 2.99 0.200 0.80 8.9 8.40 7.54 80.5 93.60.27 2.85 0.250 1.00 8.9 8.40 7.47 81.0 93.7 0.27 2.73 0.300 1.20 8.98.40 7.40 81.3 93.7 0.22 2.58 Pre-Mix 0.050 0.25 12.0 8.50 7.86 78.392.8 0.31 3.41 0.075 0.38 12.0 8.50 7.81 79.2 93.2 0.24 3.33 0.100 0.5012.0 8.50 7.70 79.7 93.3 0.22 3.19 0.150 0.75 12.0 8.50 7.54 80.4 93.60.17 3.03 0.200 1.00 12.0 8.50 7.44 80.3 93.4 0.15 2.93 0.250 1.25 12.08.50 7.37 80.7 93.5 0.15 2.77 0.300 1.50 12.0 8.50 7.28 81.0 93.6 0.122.72 Pre-Mix 0.050 0.30 15.0 8.41 7.79 77.5 92.3 0.68 3.17 0.075 0.4515.0 8.41 7.69 79.3 93.1 0.40 3.04 0.088 0.53 15.0 8.41 7.66 79.5 93.20.37 3.02 0.100 0.60 15.0 8.41 7.59 80.0 93.4 0.36 2.94 0.150 0.90 15.08.41 7.43 80.6 93.5 0.27 2.74 0.200 1.20 15.0 8.41 7.32 80.0 93.3 0.32.75 0.300 1.80 15.0 8.41 7.13 80.9 93.5 0.26 2.53 0.500 3.00 15.0 8.416.99 81.3 93.6 0.24 2.32

Example 2

[0030] Mill Trial

[0031] The results comparing the pre-mix process and the SBS pretreatedpre-mix process are presented in Table 5. It was demonstrated that theSBS pretreated pre-mix performance is superior to pre-mix alone and thehigher molar ratio (12:1) pre-mix run is also better than the pre-mixrun at a molar ratio of 9.4:1.

[0032] Pre-mix Bleaching Process:

[0033] Based on the ChromaClear™ solution dosage for the bleachingapplication and the selected molar ratio during the pre-mix process thesodium bisulfite dosage was determined. The molar ratio of(bisulfite-hydroxide)/borohydride in the pre-mix process withoutbisulfite pre-treatment varied from 9.4 to 12.0 during the trial. Therequired flow of the ChromaClear™ solution and sodium bisulfite solutionwas calculated based on the tonnage rate of the pulp bleached withpre-mix solution. The ChromaClear™ solution was supplied in poly totesand the sodium bisulfite solution was stored in a tank truck. Both theChromaClear™ and sodium bisulfite solutions were controlled by variablespeed drives and adjusted manually. The flow rate was checked using acalibration column type setup. ChromaClear™ solution was diluted toabout 2% of its original concentration and the sodium bisulfite solutionwas diluted to about 5% bisulfite. The diluted ChromaClear™ solution inthe main stream was blended with diluted sodium bisulfite solution inthe side stream in a T-type connection. The chemicals flowed through aKoflo™ in-line static mixer (model 1-40-2-6-2, 6 elements, 1 inch (2.54cm) diameter, and 9 inch (22.9 cm) length) just prior to injecting thepulp on the discharge of the 2nd stage washer. The distance between theT-type connection, at which the bisulfite solution side stream wasintroduced into the borohydride stream, and the static mixer was lessthan 1 feet (0.30 m) (estimated). The distance between the static mixerand the bleaching injection point was about 6 feet (1.8 m) (estimated).

[0034] Base-line data were collected first without any pretreatment.Immediately following the baseline data collection, the SBS pretreatmentwas started and the pre-mix process was run at a 9.4:1 molar ratio after90-120 minutes and the results compared with the base line. In anothertrial, the pre-mix process without bisulfite pre-treatment was run at a12:1 molar ratio. The ChromaClear™ solution dosage was 0.16-0.18% onbone dry fiber. Brightness was measured on the unbleached samplecollected at the brown stock tank and the bleached pulp sample collectedjust before the pulp entering the third washer. The retention time wasabout 90-120 minutes at 170° F. Samples were collected every 60 minutesduring the trial. The data in the table were averaged during the trialrun (refer to Table 5). TABLE 5 Mill trial SBS:CC Bright. Bright. MolarRun Inlet Outlet Brightness Process CC, % SBS, % Ratio¹ Time % ISO % ISOGain Pre- 0.18 0.75 9.4:1 10 hrs 61.1 68.8 7.7 mix Pre- 0.18 0.75 9.4:110 hrs 60.0 69.7 9.7 SBS² + Pre-mix High 0.17 0.85  12:1 27 hrs 62.471.4 9.0 SBS:CC ratio Pre-mix

1. A method for bleaching and color stripping recycled fibers; saidmethod comprising steps of: (a) adding sodium bisulfite to a recycledfiber slurry to produce a bisulfite-treated slurry; and (b) adding tothe bisulfite-treated slurry a mixture produced by combining: (i) anaqueous solution comprising sodium borohydride and sodium hydroxide; and(ii) an aqueous solution comprising sodium bisulfite.
 2. The method ofclaim 1 in which an amount of bisulfite used for pretreatment rangesfrom 0.025% to 5% based on weight of dry fiber.
 3. The method of claim 2in which a ratio of (moles bisulfite-moles hydroxide)/moles borohydridecombined to produce said mixture is from 0 to
 25. 4. The method of claim3 in which an amount of sodium borohydride used to produce said mixtureis from 0.003% to 0.12%, based on the weight of dry fiber.
 5. The methodof claim 4 in which a ratio of (moles bisulfite-moles hydroxide)/molesborohydride combined to produce said mixture is from 3 to
 15. 6. Amethod for bleaching and color stripping recycled fibers; said methodcomprising combining: (i) an aqueous solution comprising sodiumborohydride and sodium hydroxide; and (ii) an aqueous solutioncomprising sodium bisulfite, to produce a combined solution, and addingsaid combined solution to an aqueous slurry of recycled fibers; whereina ratio of (moles bisulfite - moles hydroxide)/moles borohydride is from0 to
 25. 7. The method of claim 6 in which an amount of sodium bisulfiteadded is from 0.05% to 5%.
 8. The method of claim 7 in which the ratioof (moles bisulfite-moles hydroxide)/moles borohydride is from 2 to 7 orfrom 9 to
 16. 9. The method of claim 8 in which an amount of sodiumborohydride added is from 0.003% to 0.12%, based on the weight of dryfiber.
 10. The method of claim 9 in which the ratio of (molesbisulfite-moles hydroxide)/moles borohydride is from 9 to 16.